1. Field of the Invention
The present invention relates to a speaker and a drive device therefor, and more particularly to a novel speaker which improves all defects of a so-called dynamic speaker and a drive device therefor.
2. Related Background Art
Most speakers currently used are so called dynamic speakers of electro-magnetic induction type and only limited high fidelity type speakers are of a static type.
The dynamic speaker has history of over eighty years and a basic principle thereof is based on an electro-magnetic induction phenomenon. Namely, when a current is produced in a magnetic field, a magnetic field is generated in accordance with an amount of current and a force results. A diaphragm of the speaker is driven by this force. In other words, when a sound current flows through a voice coil in a given magnetic field, a speaker motor is driven in accordance with Flemming""s lefthand law.
FIG. 1 shows a schematic construction of the dynamic speaker of this type. Numeral 1 denotes an edge member and numeral 2 denotes a cone sheet. Those form a diaphragm plate. Numeral 4 denotes a spider, numeral 5 denotes a voice coil, numeral 6 denotes a magnet and numeral 7 denotes a housing. The voice coil 5 is hung in the housing by the spider 4. When a current flows through the voice coil 5 in accordance with the sound signal, the voice coil 5 is vibrated and the cone sheet 2 is vibrated thereby to reproduce sound.
However, the prior art speaker has the following defects.
(1) Counter emf of dynamic motor
As shown in FIG. 1, when the conductor or the voice coil 5 moves in the magnetic field, a counter action of the counter emf is naturally generated in accordance with Flemming""s righthand law. The problem of the counter emf is combined with an impedance of the amplifier in a complex manner so that the linearity of the dynamic motor is damaged and the speaker is adversely affected. In a so-called ribbon type speaker which is a modification of the dynamic type, a similar problem occurs.
(2) Resonance phenomenon
A resonance frequency inherent to the speaker is determined by a mass of a movable part of the speaker and an equivalent mass when surrounding air is swung. A lowest resonance frequency which is normally referred to as f0 indicates a lowest reproducible frequency of the speaker. Assuming that a sound input is a sine wave having a frequency f0, the counter emf combined with the amplifier functions as an electrical damper but a disturbance of phase and a change in impedance result in and an overall input/output characteristic of the speaker is non-linear.
As shown in FIG. 1, the spider 4 and the edge member 1 attached to the housing 7 of the speaker link the movable parts such as the voice coil 5 and the cone sheet 2 to the main body. They function as a mechanical damper to serve to control Q at the resonance point (quality factor representing sharpness of resonance). However, the mechanism as the passive mechanical damper slows the overall response of the speaker. Such mechanical damper is designed from the overall consideration of the control of Q, the control of residual vibration, the response characteristic and the linearity.
(3) Group delay phenomenon
The sound signal is normally processed as a voltage signal. Voltages in a voltage range from a voltage of mV order produced by a microphone to a voltage of several volts inputted to a main amplifier do not cause a phase shift in the voltage amplification. However, since the main amplifier drives the speaker, it outputs a power or (voltagexc3x97current). When the dynamic speaker is driven by the output of the main amplifier, a low frequency sound delay phenomenon called a group delay phenomenon takes place. Simply, when the sound input is inputted to the main amplifier from the microphone output, the waveforms are the same and only the voltages are different. On the other hand, the sound output from the dynamic speaker is reproduced with more delay as the frequency becomes lower and the reproduced sound may be far from the original sound waveform.
In order to solve the above problem, it may be possible to match gains at the respective frequencies by using a Fourier transform but it is mere averaging on a time axis. It is impossible, in principle, to reproduce the original sound waveform as it is by the prior art dynamic speaker. On the other hand, it has been reported that human audible sense more sensitively senses phase information as the sound frequency is lower. In general, it is considered that the phase information of the sound of up to 1.5 KHz may be discriminated. Accordingly, it is considered that the discrimination ability is higher as the sound frequency is lower and it may be said that the group delay characteristic is a fatal defect that the prior art speaker possesses.
It is an object of the present invention to provide a novel speaker which solves defects that the prior art speaker possesses.
In order to achieve the above object, the speaker of the present invention adopts a construction to drive a diaphragm by a vibration wave motor which is driven by a travelling vibration wave. In a preferred embodiment to implement the invention, a conversion circuit for converting a sound signal to a velocity signal related to a velocity of the vibration wave motor is provided and the vibration wave motor is driven in accordance with the velocity signal from the conversion circuit.
By the above arrangement, a counter emf is not generated because the vibration wave motor does not use the electromagnetic induction. Since the vibration wave motor has a large mass relative to the movable part and contact-drives it, a static friction coefficient is large and no resonance phenomenon takes place. Further, because of voltage drive, no group delay phenomenon takes place. Accordingly, a speaker of an excellent low frequency characteristic or high fidelity is provided.